1. Field of the Invention
This invention relates to a pressure-measuring sheet, and a method for measuring pressures using this sheet. More specifically, the invention relates to a pressure-measuring sheet coated with a mixture of at least two groups of microcapsules having a volume average particle size and a number average wall thickness in a specified relationship.
2. Description of the Prior Art
There has been a need for dynamic methods of measuring various planar and linear pressures such as the clamping pressures of bolts in general machinery, pressures applied to automobile braking devices, mold pressures, tightening pressures, pressures between resins in molds and mold walls, and molding pressures within molds in the molding of thermoplastic or thermosetting resins, the abutting planar pressures of springs, the clamping planar pressures of joint parts in carburetors, fuel pipes, fuel tanks, etc., of a fuel system, collision pressures between a model of the human body and objects in an automobile collision, the linear and planar pressures of rubber and plastic rolls, the checking of the effective limit and the state of effectiveness of gasket seals, the hydraulic pressures of braking devices, the teeth abutting pressures of gears, contact planar pressures between solids, bolt clamping pressures, abutting pressures of cocks in cock valves, abutting pressures between curved surfaces, abutting pressures of steel-rolling rolls during operation, the pressure distribution on the soles of the feet and on the floor surface, and the pressure distribution between the human body and a chair. However, no simple method for measuring such pressures is presently available, and those available involve very complicated procedures.
Conventional methods for measuring pressures such as planar and linear pressures include, for example, methods which use a strain gauge based on the relationship between stress and strain, and methods which use a load meter or a pressure-sensitive paint (strain-sensitive lacquer).
When a strain gauge is used to measure planar and linear pressures, not only is a sophisticated apparatus including an amplifier, a detector, a recorder, etc., needed but also skilled techniques in operating these devices and complicated calculations involving material dynamics are required. Moreover, because of the inherent characteristics of such a method, the material against which a pressure is to be measured must have a smooth surface. (A strain gauge is described, for example, in J. Yarnell, Strain Gauge, London (1951).)
Methods using pressure-sensitive paints (strain-sensitive lacquers) have the defect that a coating unevenness tends to occur, difficulty in adhesion between the paint and the material against which a pressure is to be measured is observed, and the operation is complicated.
Methods using a load meter are unsuitable for measuring planar and linear pressures because the measuring device cannot be made small in size.
A method for measurement of pressure using a pressure-sensitive laminate is also known (e.g., as described in U.S. Pat. No. 3,647,504). This method involves the use of a one-sheet type recording sheet coated with microcapsules of different wall thicknesses which contain marking solutions of different colors. Different microcapsules are ruptured corresponding to different pressures, and therefore, the colors from the ruptured microcapsules are different. The magnitude of the pressures are determined by an examination of the colors formed.
In this method for examining pressures by changes in color, the magnitude of the pressures can be determined qualitatively, but it is difficult to measure pressures accurately. Furthermore, in practical applications, the pressure-sensitive laminate is highly colored, and may soil the hands or wearing apparel.
U.S. Pat. No. 3,469,439, on the other hand, discloses a pressure-measuring system containing a mixture of groups of microcapsules whose ranges of pressure measurement do not overlap each other. This system, however, has a number of defects among which are:
(1) A linear change in density corresponding to the pressure cannot be obtained since the pressure-measuring ranges do not overlap each other.
(2) A measuring sheet capable of covering a given range of pressures cannot be obtained by mixing a variety of groups of microcapsules obtained using conventional encapsulating methods.
(3) As a result of merely mixing a variety of groups of microcapsules having completely different ranges of pressure-measurement, pressures between the pressure measuring ranges of any two groups of microcapsules give rise to the same color density as the saturation color density of the groups of microcapsules which forms a color at a lower pressure, and therefore, pressure cannot be measured.
With the above-described background in mind, a series of investigations have been made on a method for measuring the pressures of various kinds of measuring sites without a device of a large size, highly sophisticated operating techniques and complicated calculations being involved. These investigations led to the discovery that a method which comprises contacting a recording sheet with an object to be measured to form a color under pressure, and reading the pressure of the object from the change in the optical density of the colored image on the recording sheet, the color-formation of the record sheet being effected by the contacting of a color former in a microcapsule layer and a color developer in a color developer layer in the recording sheet under pressure.
When pressure-sensitive recording sheets or pressure-sensitive copying sheets (e.g., as disclosed in U.S. Pat. Nos. 2,712,507, 2,730,456, 2,730,457, 3,418,250 and 3,425,327) are used, the accuracy of pressure measurement within a pressure range of about 50 kg/cm.sup.2 to about 1,000 kg/cm.sup.2 is poor. Investigations were continued in order to eliminate this defect, and a pressure-measuring sheet having a superior accuracy for measuring pressures of about 50 kg/cm.sup.2 to about 1,000 kg/cm.sup.2 was discovered. This discovery led to the accomplishment of the present invention.